feat(mcp): record safe tool input names by gesh · Pull Request #989 · PostHog/posthog-python

gesh · 2026-09-29T12:42:04Z

💡 Motivation and Context

MCP server owners need to measure which input names agents use. This must not expose argument values or unknown field names.

This PR ports the input-name and alias contract from @posthog/mcp 0.19.0 and 0.20.0. It adds $mcp_input_keys, $mcp_input_aliases_used, and a helper for custom dispatchers.

💚 How did you test it?

uv run pytest -q posthog/test/mcp --timeout=30
uv run ruff check posthog/mcp posthog/test/mcp/test_tool_input.py posthog/test/mcp/test_features_m4.py
uv run mypy --no-site-packages --config-file mypy.ini posthog/mcp | uv run mypy-baseline filter
make public_api_check

📝 Checklist

I reviewed the submitted code.
I added tests to verify the changes.
I updated the docs if needed.
No breaking change or entry added to the changelog.

If releasing new changes

Ran sampo add to generate a changeset file

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Codex implemented the agreed Python port. The implementation keeps schemas isolated by session and redacts unknown names by default.

Created with PostHog Desktop

github-actions · 2026-09-29T12:44:59Z

posthog-python Compliance Report

Date: 2026-10-01T16:07:50.736753+00:00
Duration: 260188ms

✅ All Tests Passed!

121/121 tests passed

Capture_V1 Tests

✅ 95/95 tests passed

View Details

Test	Status	Duration
Endpoint And Method.Targets V1 Endpoint	✅	519ms
Endpoint And Method.Does Not Use Legacy Endpoints	✅	512ms
Required Headers.Has Authorization Bearer Header	✅	512ms
Required Headers.Has Content Type Json	✅	513ms
Required Headers.Has Posthog Sdk Info Format	✅	512ms
Required Headers.Has Posthog Attempt Header	✅	512ms
Required Headers.Has Posthog Request Id	✅	511ms
Required Headers.Has Posthog Request Timestamp	✅	512ms
Required Headers.Has User Agent	✅	511ms
Body Format.Body Has Created At And Batch	✅	511ms
Body Format.No Api Key In Body	✅	511ms
Body Format.No Sent At In Body	✅	513ms
Event Format.Event Has Required Root Fields	✅	510ms
Event Format.Event Uuid Is Valid	✅	512ms
Event Format.Event Timestamp Is Rfc3339	✅	511ms
Event Format.Non Utc Event Timestamp Is Converted To Utc	✅	520ms
Event Format.Distinct Id Is String	✅	511ms
Event Format.Distinct Id At Root Not Properties	✅	511ms
Event Format.Custom Properties Preserved	✅	512ms
Event Format.Set Properties Preserved	✅	511ms
Event Format.Set Once Properties Preserved	✅	512ms
Event Format.Groups Properties Preserved	✅	512ms
Event Format.Sdk Generates Uuid If Not Provided	✅	512ms
Event Format.Event Has Required Root Fields Batch	✅	515ms
Event Format.Event Uuid Is Valid Batch	✅	516ms
Event Format.Event Timestamp Is Rfc3339 Batch	✅	517ms
Event Format.Distinct Id Is String Batch	✅	518ms
Event Format.Distinct Id At Root Not Properties Batch	✅	516ms
Event Format.Custom Properties Preserved Batch	✅	516ms
Event Format.Set Properties Preserved Batch	✅	519ms
Event Format.Set Once Properties Preserved Batch	✅	517ms
Event Format.Groups Properties Preserved Batch	✅	516ms
Event Format.Sdk Generates Uuid If Not Provided Batch	✅	516ms
Batch Behavior.Multiple Events In Single Batch	✅	521ms
Batch Behavior.Batch Envelope Smoke	✅	519ms
Batch Behavior.Flush With No Events Sends Nothing	✅	507ms
Batch Behavior.Flush At Triggers Batch	✅	1016ms
Batch Behavior.Created At Reflects Batch Creation Time	✅	511ms
Deduplication.Generates Unique Uuids	✅	521ms
Deduplication.Different Events Same Content Different Uuids	✅	516ms
Deduplication.Preserves Uuid On Retry	✅	6517ms
Deduplication.Preserves Timestamp On Retry	✅	6521ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	6524ms
Deduplication.No Duplicate Events In Batch	✅	523ms
Header Behavior On Retry.Attempt Header Starts At One	✅	512ms
Header Behavior On Retry.Attempt Header Increments On Retry	✅	13532ms
Header Behavior On Retry.Request Id Preserved On Retry	✅	6522ms
Header Behavior On Retry.Different Requests Have Different Request Ids	✅	3023ms
Header Behavior On Retry.Request Timestamp Changes On Retry	✅	6521ms
Response Format Validation.Success Response Has Uuid Keyed Results	✅	513ms
Response Format Validation.Success Response Has Ok For Each Event	✅	518ms
Response Format Validation.Success No Retry After When All Ok	✅	514ms
Response Format Validation.Success Retry After Present When Retry Events	✅	1518ms
Response Format Validation.Success No Retry After When Drop Only	✅	514ms
Response Format Validation.Response Echoes Request Id	✅	511ms
Retry Behavior.Retries On 408	✅	6523ms
Retry Behavior.Retries On 500	✅	6518ms
Retry Behavior.Retries On 503	✅	8525ms
Retry Behavior.Retries On 504	✅	6522ms
Retry Behavior.Retryable Errors Have Retry After	✅	3517ms
Retry Behavior.Respects Retry After On Retryable Error	✅	11526ms
Retry Behavior.Does Not Retry On 400	✅	2514ms
Retry Behavior.Does Not Retry On 401	✅	2516ms
Retry Behavior.Does Not Retry On 402	✅	2515ms
Retry Behavior.Does Not Retry On 413	✅	2516ms
Retry Behavior.Does Not Retry On 415	✅	2516ms
Retry Behavior.Non Retryable Errors Have No Retry After	✅	2519ms
Retry Behavior.Implements Backoff	✅	22534ms
Retry Behavior.Max Retries Respected	✅	22529ms
Partial Batch Handling.Handles 200 Full Success	✅	2513ms
Partial Batch Handling.Handles 200 With All Ok	✅	3524ms
Partial Batch Handling.Does Not Retry Dropped Events	✅	3517ms
Partial Batch Handling.Does Not Retry Limited Events	✅	3519ms
Partial Batch Handling.Prunes Ok Events On Partial Retry	✅	6522ms
Partial Batch Handling.Prunes Dropped Events On Partial Retry	✅	6524ms
Partial Batch Handling.Retries Only Retry Events From Partial	✅	6527ms
Partial Batch Handling.Partial Retry Preserves Uuids	✅	6521ms
Partial Batch Handling.Partial Retry Attempt Header Increments	✅	6520ms
Partial Batch Handling.Partial Retry Request Id Preserved	✅	6525ms
Partial Batch Handling.Respects Retry After On Partial	✅	8522ms
Partial Batch Handling.Unknown Result Treated As Terminal	✅	3518ms
Partial Batch Handling.Mixed Ok Drop Limited No Retry	✅	3520ms
Compression.Sends Gzip Content Encoding	✅	513ms
Compression.No Content Encoding When Disabled	✅	511ms
Compression.Compressed Body Is Decompressible	✅	512ms
Error Handling.Does Not Retry On Unknown 4Xx	✅	2512ms
Event Options.Cookieless Mode Override	✅	512ms
Event Options.Disable Skew Correction Override	✅	512ms
Event Options.Process Person Profile Override	✅	511ms
Event Options.Product Tour Id Override	✅	511ms
Event Options.Unset Options Omitted	✅	511ms
Event Options.Options Override In Batch	✅	515ms
Geoip And Historical Migration.Geoip Disable Injected Into Properties	✅	511ms
Geoip And Historical Migration.Historical Migration Set In Body	✅	512ms
Geoip And Historical Migration.Historical Migration Absent By Default	✅	511ms

Capture_Ai Tests

✅ 5/5 tests passed

View Details

Test	Status	Duration
Routing.Capture Ai Posts To Ai Endpoint	✅	511ms
Routing.Capture Does Not Reroute Ai Named Events	✅	512ms
Identity.Capture Ai Event Has Uuid	✅	511ms
Identity.Capture Ai Keeps Supplied Uuid	✅	512ms
Timestamp Format.Non Utc Event Timestamp Is Converted To Utc	✅	513ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	13ms
Request Payload.Flags Request Uses V2 Query Param	✅	12ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	12ms
Request Payload.Flags Request Omits Authorization Header	✅	12ms
Request Payload.Token In Flags Body Matches Init	✅	12ms
Request Payload.Groups Round Trip	✅	13ms
Request Payload.Groups Default To Empty Object	✅	12ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	12ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	12ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	12ms
Request Lifecycle.No Flags Request On Init Alone	✅	4ms
Request Lifecycle.No Flags Request On Normal Capture	✅	511ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	18ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	12ms
Retry Behavior.Retries Flags On 502	✅	316ms
Retry Behavior.Retries Flags On 504	✅	317ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	514ms

Feature_Flags_Local_Evaluation Tests

✅ 4/4 tests passed

View Details

Test	Status	Duration
Versioned Boolean Matching.Matching Version Missing	✅	80ms
Versioned Boolean Matching.Matching Version 1	✅	77ms
Versioned Boolean Matching.Matching Version 2	✅	76ms
Versioned Boolean Matching.Version Only Reload 1 2 1 2 Missing	✅	46ms

greptile-apps · 2026-09-29T12:46:29Z

[Medium risk] Adds safe input field name recording to MCP tool-call events.

The PR should not merge until declared input names are captured for referenced standalone FastMCP schemas.

_{Reviews (2) · Last reviewed commit: "fix(mcp): scope input schemas to tool ca..."}

Generated-By: PostHog Desktop Task-Id: dda38523-5b8d-47b6-a11a-64cd59a812fe

lucasheriques

The no-values design and the analytics-key filtering hold up. Approving, with a few parity questions, since the same property would carry different data in Python and JS.

lucasheriques · 2026-10-01T16:47:09Z

+    assert properties["$mcp_input_keys"] == ["a", "b"]
+
+
+async def test_lowlevel_does_not_reuse_a_listed_schema_for_input_names():


question: Raw low-level servers always record ["[redacted]"], even after a listing, and this test locks that in. JS uses listed schemas per session here. I'm guessing 57a3d9a followed the same reasoning as _standalone_injected_parameters (listings can differ across requests). For names, though, a stale schema only risks recording a name the server declared, not a private one. Was there another concern?

lucasheriques · 2026-10-01T16:47:09Z

+    ]
+
+
+def _aliases_used(


question: This differs from JS describeAliasesUsed, which takes the first alias present and never calls should_record_input_key. Here a too-long alias falls through to a later one, and the rule runs on the canonical name (absent from the input, with declared=True). Same input, different $mcp_input_aliases_used per SDK. Match JS, or land the stricter rule there too? Matching JS also removes recording_decisions.

lucasheriques · 2026-10-01T16:47:09Z

        model_ours = data.tool_model_parameter_injected.get(name, model_injectable)
        if _dispatch_can_differ(high_level):
            model_ours = False
+            input_schema = None


question: With _dispatch_can_differ, any middleware (even FastMCP's LoggingMiddleware) redacts every name: ['a', 'b'] becomes ['[redacted]']. The v2 path handles middleware by resolving through server.list_tools() in the request. Could 1.x do the same for input names?

lucasheriques · 2026-10-01T16:47:09Z

    return declared, injectable


+def _input_schema_view(


question: Reading the code, $ref dereferencing only runs on the mcp 1.x path, while v2 passes raw tool.parameters. Would a root-$ref schema redact everything under mcp 2.x? If so, moving it into _declared_properties would cover every adapter.

lucasheriques · 2026-10-01T16:47:09Z

    return properties if isinstance(properties, dict) else {}


+def _tool_input_schema_v2(high_level: Any, name: str) -> Optional[Dict[str, Any]]:


nits:

This and _tool_input_schema in fastmcp are identical. Pass the schema to start_tool_call_lifecycle in every adapter and drop the replace calls.

README: document should_record_input_key, the 20/64 limits, and that $mcp_parameters still carries raw names.

The helper tests could be one parametrize table, like JS's it.each.

gesh self-assigned this Sep 29, 2026

greptile-apps Bot reviewed Sep 29, 2026

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Comment thread posthog/mcp/_instrumentation.py Outdated

Comment thread posthog/mcp/_instrumentation.py Outdated

Comment thread posthog/mcp/_tool_input.py Outdated

Comment thread posthog/mcp/_tool_input.py Outdated

gesh force-pushed the posthog/mcp-input-names branch from 2f602c9 to 57a3d9a Compare October 1, 2026 14:55

gesh marked this pull request as ready for review October 1, 2026 15:07

gesh requested a review from a team as a code owner October 1, 2026 15:07

gesh requested a review from a team October 1, 2026 15:08

greptile-apps Bot reviewed Oct 1, 2026

View reviewed changes

Comment thread posthog/mcp/_instrument_lowlevel.py Outdated

gesh added 3 commits October 1, 2026 19:02

feat(mcp): record safe tool input names

c6d9e07

Generated-By: PostHog Desktop Task-Id: dda38523-5b8d-47b6-a11a-64cd59a812fe

fix(mcp): scope input schemas to tool calls

187347c

fix(mcp): resolve referenced input schemas

c520c54

Generated-By: PostHog Desktop Task-Id: dda38523-5b8d-47b6-a11a-64cd59a812fe

gesh force-pushed the posthog/mcp-input-names branch from c8b2409 to c520c54 Compare October 1, 2026 16:02

lucasheriques approved these changes Oct 1, 2026

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feat(mcp): record safe tool input names - #989

feat(mcp): record safe tool input names#989
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		assert properties["$mcp_input_keys"] == ["a", "b"]


		async def test_lowlevel_does_not_reuse_a_listed_schema_for_input_names():

		return properties if isinstance(properties, dict) else {}


		def _tool_input_schema_v2(high_level: Any, name: str) -> Optional[Dict[str, Any]]:

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gesh commented Sep 29, 2026

💡 Motivation and Context

💚 How did you test it?

📝 Checklist

If releasing new changes

🤖 Agent context

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posthog-python Compliance Report

✅ All Tests Passed!

Capture_V1 Tests

Capture_Ai Tests

Feature_Flags Tests

Feature_Flags_Local_Evaluation Tests

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